Cardiac stimulation devices, such as pacemakers, implantable cardioverter defibrillators (ICD's) or congestive heart failure devices (CHF) play an important role in many cardiac patient lives. These devices typically monitor cardiac arrhythmias, and in most cases, provide a form of electrical stimulation therapy to the heart as needed. Modern electronics have enabled the miniaturization of these devices for implantation in a patient for constant monitoring while the patient lives a normal life.
One of the features of a cardiac stimulation device (CSD) is the ability to apply a stimulation pulse across a desired portion of the cardiac tissue, and achieve a specific response in the form of a desired contraction of the heart tissue in that area. The devices typically employ a pulse generator disposed in a metallic housing, and one or more leads coupled to the output of the generator. Each lead carries one or more electrodes for directing the pulse across the desired cardiac tissue. Consequently, the overall construction of the pulse generator output circuitry, the lead configurations, and the electrode configurations are important for maximum flexibility in positioning and configuring the leads and electrodes.
An undesirable side-effect often associated with conventional atrial cardioversion and ventricular defibrillation involves discomfort experienced by the patient during pulse therapy. Much of the discomfort results from current conduction through non-cardiac tissue along shock vectors directed partially outside the heart. While the patient is often unconscious during ventricular defibrillation, often rendering the discomfort issue moot, this is not typically the case for low energy pulse therapies.
FIG. 1 illustrates some available shock vectors associated with conventional atrial cardioversion techniques. Shock vectors are generally defined according to electrode placement. Typically, electrodes 20, 22 and 24 are disposed in the right atrium (RA), right ventricle (RV), and coronary sinus or other cardiac vein. The metallic housing of a cardiac stimulation device 26 often serves as a return electrode. A first current conduction path, or current vector CV1, is defined by the RA electrode and the return electrode. This path may be problematic for patient comfort reasons because pectoral tissue lies within the vector. A second current vector CV2, defined between the RA coil and the coronary sinus electrode would be an attractive vector were it not for the difficulty in placing and maintaining the electrode in the desired position.
Further referring to FIG. 1, the use of the RV electrode 22 as the return electrode serves to define a more attractive current vector than those described above. This is because a large portion of the current flows through the heart, along current vector component CV3. However, due to the low position of the RV coil, a substantial portion of the current exits the heart as shown with current vector component CV4. This often causes shock pain to the patient. Moreover, the low (actually referred to as the “apex”) position of the conventional RV electrode during atrial cardioversion typically wastes energy by unnecessarily depolarizing the ventricles during delivery of the pulse between the RV electrode 22 and the RA electrode 20.
For ventricular defibrillation, a conventional shock vector SV1 is shown in FIG. 2, generally emanating from the single RV electrode 22 to the housing of the cardiac stimulation device 26 along a path of least resistance. Typically, the device generates a very large shock on the order of around four-hundred to eight-hundred volts between the electrode and the housing in an effort to stimulate enough of the heart tissue sufficient to terminate the fibrillation. Unfortunately, the conventional shock vector components tend to bypass the opposite ends of the septal wall, at 30 and 32 (in phantom), separating the right ventricle from the left ventricle. In some cases, enough cardiac tissue in these areas remains polarized to maintain the fibrillation activity, possibly necessitating prolonged shock therapy. Moreover, the shock magnitude often causes lingering pain for the patient after the restoration of consciousness.
A proposal that allegedly minimizes cardiac stimulation pain is described in U.S. Pat. No. 6,205,357 to Idecker et al. This proposal employs a plurality of electrodes, with at least one of the electrodes being disposed on the surface of the left ventricle. The pulse therapy utilizes a sequencing technique for delivering pulses with the left ventricular electrode. While this technique appears beneficial for its intended applications, it has drawbacks. For instance, in many circumstances the placement of the left ventricular electrode is very difficult. Moreover, maintaining the electrode in a stable position is also challenging.
In an effort to avoid the use of a left ventricular electrode for ventricular resynchronization, researchers have described a way of stimulating the left ventricle with a pair of right ventricular electrodes. The electrodes are placed such that a first electrode sits in the high basal region of the right ventricle, and a second electrode resides at the apex. Although the electrodes lie at different heights in the left ventricle, both are disposed proximate the septum at opposite ends. In employing this bifocal electrode arrangement, and applying specific pulses at appropriate timings, resynchronization of the ventricles may be accomplished in a manner similar to other resynchronization techniques that employ a left ventricular electrode.
What is needed and currently unavailable is a cardiac stimulation system and associated methods that deliver pulse therapies for both atrial and ventricular arrhythmias with minimal pain, and maximum efficiency. The system and methods described herein satisfy these needs.